Stand age, degree of encroachment and soil characteristics modulate changes of C and N cycles in dry grassland soils invaded by the N2-fixing shrub Amorpha fruticosa

The N₂-fixing shrub Amorpha fruticosa L. is rapidly spreading in the dry riparian natural grasslands of Europe, altering ecosystem functions and depleting plant diversity. Alteration of the N cycle represents the key factor involved in invasions by N₂-fixing plants with cascading effects on plant species richness. We hypothesized that A. fruticosa encroachment strongly impacts not only the N but also the C cycle and that the magnitude of such alterations may be modulated by soil characteristics. To test these hypotheses, we selected four river floodplains in North East of Italy and compared natural uninvaded grasslands with half invaded and completely invaded sites, based on A. fruticosa stand characteristic and relevant leaf traits and on soil properties related to soil texture and to C and N cycles. Soil organic matter mineralisation, ammonification and nitrification rates were determined. Soil nitrification increased remarkably with plant invasion while ammonification was significantly higher only in half invaded sites. Soil organic matter mineralisation, microbial biomass C sustained per soil organic C unit and nitrification positively correlated with stand age, regardless to the stage of the encroachment. Mineralisation and nitrification increased with soil organic C and total N in uninvaded and completely invaded sites, but decreased in half invaded sites. At the half invasion stage, trends in nitrification and CO₂ mineralisation were transitionally reverted and remediation may be facilitated by less pronounced changes in soil properties compared to completely invaded sites. Direct effects of plant invasion are modulated by the action of soil characteristics such as soil organic C and clay contents, with soils rich in organic C showing larger nitrification and mineralisation rates.

Agricultural land use curbs exotic invasion but sustains native plant diversity at intermediate levels

Unveiling the processes driving exotic plant invasion represent a central issue in taking decisions aimed at constraining the loss of biodiversity and related ecosystem services. The invasion success is often linked to anthropogenic land uses and warming due to climate change. We studied the responses of native versus casual and naturalised exotic species richness to land uses and climate at the landscape level, relying on a large floristic survey undertaken in North - Eastern Italy. Both climate and land use drove exotic species richness. Our results suggest that the success of plant invasion at this scale is mainly due to warm climatic conditions and the extent of urban and agricultural land, but with different effects on casual and naturalized exotic species. The occurrence of non-linear trends showed that a small percentage of extensive agricultural land in the landscape may concurrently reduce the number of exotic plant while sustaining native plant diversity. Plant invasion could be potentially limited by land management, mainly focusing on areas with extensive agricultural land use. A more consciousness land management is more and more commonly required by local administrations. According to our results, a shift of intensive to extensive agricultural land, by implementing green infrastructures, seems to be a win-win solution favouring native species while controlling the oversimplification of the flora due to plant invasion.

Plant growth shapes the effects of elevation on the content and variability of flavonoids in subalpine bilberry stands

The study of morphological and physiological responses of shrubs to climate is crucial for the understanding of future scenarios regarding climate change. In this light, studying shrub growth and physiological acclimation along an elevation gradient might be insightful. The phenolic metabolic pathway represents a powerful tool to interpret such processes. In the South-Eastern Alps, we investigated the relationships between elevation, plant traits (i.e. age, xylem ring width, annual shoot length), plant-plant interaction (i.e. shrub cover) and flavonoids in Vaccinium myrtillus L. (leaves, berries) in stands above the treeline. The relationships were parsed within causal networks using a confirmatory path analysis. Elevation was the main driver of V. myrtillus growth, having both direct and indirect effects on the leaf flavonoid content, but this was less evident for berries. In particular, the content of foliar flavonoids showed a peak at mid-elevation and where the growth of xylem rings was intermediate, while it decreased in stands with higher shoot length. Flavonoid content variability of both leaves and berries was affected by elevation and shoot length. In berries, flavonoid variability was further related to all growth traits and shrub cover. These findings evidence that flavonoid content is influenced by both elevation and growth traits of V. myrtillus, often showing non-linear relationships. These results suggest a trait-mediated response of this plant to climate conditions as a result of trade-offs between plant growth, plant defence, environmental stress and nutrient/resource availability.

'Last In-First Out': seasonal variations of non-structural carbohydrates, glucose-6-phosphate and ATP in tubers of two Arum species

Knowledge on the metabolism of polysaccharide reserves in wild species is still scarce. In natural sites we collected tubers of Arum italicum Mill. and A. maculatum L. - two geophytes with different apparent phenological timing, ecology and chorology - during five stages of the annual cycle in order to understand patterns of reserve accumulation and degradation. Both the entire tuber and its proximal and distal to shoot portion were utilised. Pools of non-structural carbohydrates (glucose, sucrose and starch), glucose-6-phosphate and ATP were analysed as important markers of carbohydrate metabolism. In both species, starch and glucose content of the whole tuber significantly increased from sprouting to the maturation/senescence stages, whereas sucrose showed an opposite trend; ATP and glucose-6-phosphate were almost stable and dropped only at the end of the annual cycle. Considering the two different portions of the tuber, both ATP and glucose-6-phosphate concentrations were higher in proximity to the shoot in all seasonal stages, except the flowering stage. Our findings suggest that seasonal carbon partitioning in the underground organ is driven by phenology and occurs independently of seasonal climate conditions. Moreover, our results show that starch degradation, sustained by elevated ATP and glucose-6-phosphate pools, starts in the peripheral, proximal-to-shoot portion of the tuber, consuming starch accumulated in the previous season, as a 'Last In-First Out' mechanism of carbohydrate storage.